Benchmark variational coupled cluster doubles results

We present the first application of the Rayleigh-Ritz variational procedure to the coupled cluster doubles trial function. The variational approach is applied to the potential surface of H-4, the double dissociation of water and the dissociation of N-2, and the results are compared to standard coupled cluster doubles calculations. It is found that the variational approach gives a greatly improved description of strongly correlated systems, where the standard approach is known to fail. Some examination of the basis set dependence of the results is presented. (C) 2000 American Institute of Physics. [S0021-9606(00)30244-6].